In the welding arts, the initial manufacturing process, the after-manufacture treatment of the welded product, and the encountering of and the magnitude of loads in use and the aging process cause deteriorization of load bearing strength in the structure of the materials, whether unseen without destructive analysis or evidenced by catastrophic failure, such as by appearance of fractures or cracks. This invention is directed to procedures that remove defects in the material structure and that improve performance of the welded product by relieving stress centers and stress patterns about weld seams, thereby to improve load carrying capacity, life of service and stability of parameters.
Conventional welded products are made by employment of various welding art technological operation steps before and after the actual welding step in an attempt to improve the working life of the products. Some of these steps are typified by (a) pre-welding preparation of exposed surfaces at welding sites by abrasive or chemical cleaning, (b) post-welding processing of welded seams by cleaning flux and slag and by surface shaping to remove visible sharp projections and contours that identify concentrated stress areas, (c) surface treatment of the welded structure with corrosion resisting coatings and (d) thermal tempering and demagnetizing treatments for internally restructuring the metal grain in a manner reducing the influence of stress concentrations.
Because of the interactions of the various independent steps typically occurring at various times on welded metal products, particularly in view of various intricate product shapes and loading patterns, and the difficulties in detecting defective subsurface base material patterns such as grain structure and stress centers in the welded product that affect fatigue, life and strength of welded joints and associated zones, results of combatting fatigue with various technical operations heretofore available in the prior art have been substantially erratic and unpredictable thus producing low quality products inconsistent in expected performance.
At this stage of the prior art, a number and variety of interacting technical operations in a series of processing steps in initial production are necessary to prepare respective welded metal products with greater load bearing capacity and lower internal stresses for longer expected life cycles. Simplification and lower cost of the production process is thus highly desirable, as well as improved performance.
Also current welded metal product production practices require the addition and removal of materials which therefore are consumed in the manufacturing process. For example overlay welding and beading operations for strengthening weld seams require more initial metal and require additional technical operations such as mechanical grinding, removal of fluxes, and thermal tempering. It has not been feasible to obtain optimum strength and life in welded products without such steps.
It is conventional to retire and replace aging welded structures such as steel bridgework for stress corrosion, stress fatigue and the presence of observable cracks. It is therefore desirable to provide improved maintenance and repair technology to renew useful life by restoring initial load bearing strength and relieving stress fatigue so that current structures may be kept in operation.
In the welding arts conventionally in practice, technology has not been available which is well adapted for in-use non-destructive and non-deforming repairs to restructure and restore welded products that have become overloaded from aging, that have reduced loading capacity because of stress fatigue or which have catastrophically failed by cracking or the like.
For example, the prior art ability to repair visible catastrophic failures of structure, evidenced by cracks or fractures, in most part is limited to the addition of supporting braces, crutches, and other types of overlying structure to bypass the damaged zones. Such techniques are not suitable for many utilities where there is either no accessible place to rework the welded products in-situ, where restrictions in space are imposed or where appearance of such bypassing structure is intolerable.
Accordingly it is an objective of the present invention to correct such foregoing defects of the prior art.
Examples of typical prior art technology related to the invention or teaching some of the underlying methodology merged to produce the novel combination of technologies as a whole involved in the present invention, now are set forth.
Overlay technology exists wherein strengthening elements are superimposed over critical zones to bypass fatigued, fractured or other deficient welded product structure. The overlay may be superimposed directly upon weld seams in some cases. Typical examples of this technology are U.S. Pat. No. 2,537,533, G. E. Ingalls, Jan. 9, 1951; RE 16,599 R. Mattice, Apr. 19, 1927; U.S. Pat. No. 1,703,111, S. J. Kniatt, Feb. 26, 1929; and U.S. Pat. No. 1,770,932, A. G. Leake, Jul. 22, 1930. Such overlay structure in U.S. Pat. No. 4,049,186, R. E. Hanneman, et al., Sept. 20, 1977 and U.S. Pat. No. 4,624,402, D. Pitcairn, et al., Nov. 25, 1986 in particular disclose overlay welds for the purpose of preventing stress corrosion failures in the welded body.
Peening by means of pellets, hammers, stress waves and ultrasonic impact is known to surface treat and deform the welded body surface structure for contouring weld sites and heating the metal for thermal tempering effects. Typical art of this nature includes U.S. Pat. No. 5,654,992, K. Uraki, et al., Aug. 5, 1997 and U.S. Pat. No. 3,961,739, B. P Leftheris, Jun. 8, 1976. These disclosures recognize that mechanical pressure and stress waves applied to the external surface of a body creates thermal energy and a momentary state of plasticity in the workpiece.
In U.S. Pat. No. 4,330,699, M. M. Farrow, May 18, 1982, a non-contact laser welder is accompanied by a second amplitude modulated laser for generating acoustic waves in the melt to improve interdiffusion and homogeneity of the weld joint.
I have authored or co-authored several publications relating to ultrasonic impact treatment of welded joints and the relationship to fatigue resistance, typically as reported in the following International Institute of Welding IIW Documents:
Publication XIII-1617-96 for example discloses that the fatigue strength of as-welded joints was increased by changes in mechanical properties of material in surface layers induced by ultrasonic impact treatment (UIT). Thus, the material at the weld toe is compressed and deformed by manual indentation using an ultrasonic probe to form indented groove structure smooth and free from irregularities. This technique depends upon the training and skills of an operator manually wielding an ultrasonic probe to form the grooves, and requires reshaping of the weld site.
The comparison of peening with UIT is discussed in Document XIII-1668-97, which sets forth the advantages of ultrasonic technology over peening, and the practicability of UIT technology to compress and indent the welded body structure in the vicinity of the weld seam.
The use of ultrasonic hand tools for achieving foregoing compression indentations is set forth in Document XIII-1609-95.
These techniques have demonstrated significant increases in fatigue limits of welded structures. However, this prior art technology requires physical distortion of the welded product or structure, and demands skilled labor to make decisions on the nature of indentations in the presence of different physical shapes of welded bodies and different loading requirements at the weld sites. Thus, it is neither practical nor economically feasible to apply the techniques universally to welded products of various sorts. Furthermore there can be no consistency from one product to another to assure constant quality performance expectations. Nor can this technique, provided for initial production cycles only, be used for later maintenance of welded products or for repairs of cracks and catastrophic failures.
The present invention has the objectives of curing defects and deficiencies in the aforementioned type of prior art, and offering significant advantages in simplifying processing steps and improving useful life span and higher loading capacities of welded products at various stages of life throughout the initial production of the product and even after catastrophic failures appear such as visually observable cracks.
A significant objective of the invention is to coordinate and combine non-destructive ultrasonic impact treatment of welded products without deforming shape applicable to manufacturing, maintenance and repair process stages for relaxing fatigue resistance, improving corrosion fatigue strength and durability of welded joints, and creating uniformly distributed stress gradient junctions between welding seams and the internal base metal in target areas of a welded joint.
Another objective of the invention is to reduce material consumption during welding while reducing the processing time and increasing the performance and life of welded products by replacing or eliminating various required technical operations of the prior art in the production, maintenance and repair of welded products such as grinding and surface shaping.
A more specific objective of the invention is to produce quality welded joints with consistently controlled stress distribution patterns that substantially increase useful lives and working strengths of welded products in a manner not heretofore feasible.